The invention relates to a calender for treating a product web, in particular a paper web, for example a smoothing calender.
A calender of this type is disclosed, for example, by DE-U-295 04 034. In this calender, an intermediate roll in the roll stack is usually driven and drives the other rolls along by means of friction with the product web. In the document cited, it is specified that the normally passively driven rolls are driven actively in order to thread the product web into the nips. This auxiliary drive needs to be designed only for the idling power until the operating speed is reached, whereas the main drive has to be designed for total power output during operation.
Forces that are fed in from the outside act on the rolls in the vertical direction, as does the weight, increasing from top to bottom, of the rolls mounted above. Deformations that are caused by this--in particular deflection--can be compensated for by means of the deflection controlled rolls. However, forces act on the rolls in the horizontal direction as well. These forces can be attributed to the friction-induced torque transmission mentioned, as is explained in the publication Pav/Svenka, "Der Kompaktkalander--die Antwort auf die Herausforderung nach hohen Geschwindigkeiten bei der Glattung und Satinage" [The compact calender--the answer to the challenge of higher speeds in smoothing and calendering], DAS PAPIER 1985, pp. V178 ff. In this publication, mention is also made of a compact calender, in which four resilient rolls with their own drives form nips around a hard base roll that is mounted in a stationary manner. This is intended to dispense with the interlinking of the roll set, as is unavoidable in the case of calenders of this type.
Whereas vertical deformations of the rolls, as explained above, can be compensated for, this does not apply to deformations resulting from horizontally acting forces. This means that the rolls must have minimum diameters in order that horizontal deformations can be kept within tolerable limits. One of these limitations resides in the fact that, in the event of a deformation of a roll in the horizontal direction, the distribution of the line load becomes non-uniform, the regions close to the bearings being loaded more severely. This can lead to over-pressing of the product web in the edge region and to the unequal distribution of the product-web property values in the cross-machine profile. Furthermore, increased wear of the resilient roll covers and, in the extreme case, destruction of the same can occur. At a given line load, the compressive stress is limited by the minimum diameters of the rolls to an appropriate value, which may be increased only by increasing the line load. However, even if the horizontal deformation of the rolls is kept within limits, shear stresses nevertheless act on the product web in the nip and--in the case of paper--can loosen the bonding between the fibres in the web running direction and thereby reduce the strength of the paper.
The object of the invention is to specify a calender in which the compressive stress can be increased at a given line load by the roll diameters being reduced.
Since the roll diameters no longer need to be designed in accordance with the horizontally acting forces, because the effects of the latter are controlled out, it is possible to design the rolls with diameters which are determined by criteria other than the resistance to deflection or the horizontal deformations, for example by the critical inherent frequencies.
Rolls of a smaller diameter have a lower weight, so that the static (gravitation-induced) forces are lower in relative terms and smaller bearings can also be used.
The drives apply the specific power for the respectively driven roll, this power being composed of re-forming, transporting and loss power. In this case, a distribution of 50:50 to the two nip-forming rolls would be only a rough guide, since, for example, a deflection controlled roll has considerably higher friction losses than a normal solid roll.
The forces which are to be controlled out according to the invention can be measured, for example, in the roll bearings; bearings with force-measuring systems incorporated are commercially available. However, it is at least also conceivable to use measurement methods to register the horizontal deformations that are brought about by such forces.
An exemplary embodiment of a calender according to the invention is illustrated in the appended drawings and will be explained below in detail .